Supersonic flight has been possible for more than 60 years, but is not without its challenges. One such challenge is the sonic “boom” caused on the ground as a supersonic aircraft flies over. Another challenge is that a majority of air-breathing jet engines (with the exception of scramjets) need subsonic flow through the engine to operate properly.
Most supersonic aircraft use some sort of variable inlet to create a shock system to provide subsonic flow to the engine. This shock system consists of a series of oblique shocks (not normal to the flow direction) followed by a normal shock (flow normal to the shock wave) that reduces the flow to subsonic speeds. Because supersonic aircraft must land and take off on normal runways, however, a large range of flights speeds is required. The aircraft may take off and land in the low subsonic speed range (150-220 mph), and yet cruise up to Mach 1.5 or 2.0 (1,000-1,500 mph). As a result, many aircraft use some sort of variable geometry inlet to maintain proper orientation of the shock system.
At subsonic flight speeds, for example, most inlet designs (cone, 2D ramps, etc.) operate much like a so-called “pitot” inlet in which flow is ingested without the presence of shock waves. As the vehicle goes supersonic, however, a shock wave appears, emanating from the cone or ramp. As flow passes through the shock wave, the Mach number decreases and flow is compressed. As the flight Mach number increases, however, the shock wave becomes more oblique (flattens) and eventually impinges on the cowl lip of the inlet. If the shock wave enters the inlet, flow quality is degraded to the point where the engine can stall (or “unstart”).
For higher Mach numbers, therefore, one or more moving surfaces or so-called ramps becomes necessary to maintain the shockwave at or just outside the inlet cowl lip over a wide range of speeds. With increasing flight speed, for example, the ramps must be moved in order to maintain the shock system structure at the design condition of “shock-on-lip.”
Unfortunately, due to the high pressure of supersonic flow, these variable inlet devices (e.g., cones or ramps) are subject to very high aerodynamic forces. To withstand these forces, provide the necessary responsiveness, and provide suitable service life, therefore, the variable inlet components tend to be heavy and complicated. This reduces overall aircraft efficiency, and increases production and maintenance costs.
What is needed, therefore, is a supersonic inlet with performance (flow quality) similar to what can be obtained with variable geometry but with few, or no, moving parts. The system should be simple, robust, responsive, and easy to maintain. It is to such systems and methods that embodiments of the present disclosure are primarily directed.